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Re: SR-IOV: do we need to virtualize in Xen or rely on Dom0?

  • To: Oleksandr Andrushchenko <Oleksandr_Andrushchenko@xxxxxxxx>
  • From: Jan Beulich <jbeulich@xxxxxxxx>
  • Date: Thu, 10 Jun 2021 14:02:04 +0200
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  • Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Roger Pau Monné <roger.pau@xxxxxxxxxx>
  • Delivery-date: Thu, 10 Jun 2021 12:02:12 +0000
  • List-id: Xen developer discussion <xen-devel.lists.xenproject.org>

On 10.06.2021 13:45, Oleksandr Andrushchenko wrote:
> Hi, Jan!
> On 10.06.21 13:48, Jan Beulich wrote:
>> On 10.06.2021 12:01, Oleksandr Andrushchenko wrote:
>>> On 10.06.21 10:54, Roger Pau Monné wrote:
>>>> OTOH if we properly trap accesses to the SR-IOV capability (like it
>>>> was proposed in [1] from your references) we won't have to modify OSes
>>>> that want to run as hardware domains in order to handle SR-IOV devices.
>>> Out of curiosity, could you please name a few? I do understand that
>>> we do want to support unmodified OSes and this is indeed important.
>>> But, still what are the other OSes which do support Xen + PCI passthrough?
>> I think Roger saying "want" meant to cover ones which currently don't,
>> and which would have to undergo more extensive changes if they were to
>> be enabled.
> Fair enough. Do you think we would also need to re-work the existing code
> in Xen to support normal devices (not SR-IOV), e.g. we currently rely on
> PHYSDEVOP_XXX and other Linux specifics.

Yes, work in that area would also be needed. For example we'd need to
scan buses / segments as they become accessible. Right now we only scan
segment 0, and even that's only possible because on x86 mmconfig is not
the only way to access config space.

> And even if SR-IOV is implemented
> in Xen this won't allow those OSes to stay unmodified, including FreeBSD.

Of course, it's the nature of PVH (as opposed to HVM) that OSes need
modification. The question is the scope thereof.




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